We have measured the ability of various glycoproteins to act as acceptors (substrates) for the action of GlcNAc-T V. Several of the glycoproteins are not efficient acceptors until they are denatured. After denaturation, these glycoproteins become very efficient acceptors. These results suggest that accessibility of oligosaccharide acceptors to the enzyme is a major determinant of whether a particular oligosaccharide is acted upon by GlcNAc-T V.